CRYPTOGAMS 313 



by the cells themselves. Sometimes these chains remain connected 

 and form bands or zigzag chains, or, on the other hand, they are 

 attached and enclosed in gelatinous tubes, while in the case of the 

 marine genus Schizonema they lie embedded in large numbers in a 

 gelatinous branching thallus, often over 1 dcm. in breadth. The cells 

 also display a great diversity of shape ; while generally bilaterally 

 symmetrical, they may be circular or elliptical, rod- or wedge-shaped, 

 curved or straight. The structure of their cell walls is especially 

 characteristic; it is composed of two halves or valves, one of which over- 

 laps the other like the lid of a box (Fig. 3, B, p. 11). The cells thus 

 present two altogether different views, according to the position in which 

 they are observed, whether from the girdle (Fig. 3, B) or valve-side 

 (Fig. 3, A). Both valves are so strongly impregnated with silica, that, 

 even when subjected to intense heat, they remain as a siliceous skeleton, 

 retaining the original form and markings of the cell walls. The walls 

 of the cells, particularly on the valve side, are often ornamented with 

 numerous fine, transverse markings or ribs, and also with small 

 protuberances and cavities. In many instances (Fig. 3) a longitudinal 

 line corresponding to an opening in the cell walls, and exhibiting 

 swollen nodules at both extremities and in the middle, is distinguish- 

 able in the surface of the valves. Forms provided with such a median 

 suture or RAPHE are characterised by peculiar backward-creeping 

 movements, resulting from the extrusion of protoplasmic protrusions 

 from their longitudinal edges. Each frustule has always a central 

 nucleus and one (Fig. 3) or two large or numerous smaller (Fig. 

 233, D) chromatophores embedded in its parietal protoplasm. These 

 chromatophores or endochrome plates, as they are often called, 

 are flat, frequently lobed, and of a brownish-yellow colour. In 

 addition to chlorophyll they contain a golden brown colouring matter, 

 termed diatomin. Globules of a fatty oil are also included in 

 the cell contents, and take the place of starch as an assimilation 

 product. 



The Diatomeae multiply vegetatively by bipartition, which always 

 takes place in one direction. In this process the two valves are first 

 pushed apart from one another by the increasing protoplasmic contents 

 of the mother cell, which then divides longitudinally and always in 

 such a direction that each of the two new cells retains one valve of the 

 original frustule. After the division of the protoplasm of the mother 

 cell is accomplished, each daughter cell forms, on its naked side, a 

 new valve fitting into the old one. The two valves of a cell are 

 therefore of different ages. In consequence of this peculiar manner of 

 division, as the walls of the cells are silicified and incapable of dis- 

 tension, the daughter cells become successively smaller and smaller, 

 until finally, after becoming reduced to a definite minimum size, they 

 undergo transformation into auxospores. The auxospores are usually 

 two or three times larger than the frustules from which they arise, 



